(1) Field of the Invention
The present invention relates to sheet molding compounds (SMC) as prepregs which contain biofibers as well as molded products produced from the prepregs. The present invention also relates to an apparatus for producing the mats of the biofibers and the prepregs.
(2) Description of Related Art
Natural and wood fiber plastic composites have continued their phenomenal growth in 2002. As per the recent market study by Principia Partners, the demand of these products in North America and Western Europe combined, will reach nearly 1.3 billion pounds valued at roughly $900 million. This represents a growth of almost 20% from 2001 levels. There is a growing interest in the use of natural/biofibers as reinforcing components for thermoplastics and thermosets. Although thermoplastics have the added potential advantage of recyclability, thermosets are capable of superior mechanical properties compared to thermoplastics in the resulting biocomposites. Biocomposites derived from natural fibers and petroleum-based thermoplastics or thermosets are not fully environmentally friendly because matrix resins are non-biodegradable. However these biocomposites do possess a balance between economics and environment allowing them to be considered for applications in the automotive, building, furniture and packaging industries. Natural fiber composites are mainly price-driven commodity composites that have useable structural properties at relatively low cost. Advantages of natural fibers over traditional reinforcing fibers such as glass and carbon are: low cost, low density, high toughness, acceptable specific stiffness and strength properties, enhanced energy recovery, carbon dioxide sequesterization and biodegradability.
The hydrophilic nature of biofibers is a potential cause for concern in compatibility, adhesion and dispersion problems with hydrophobic polymer matrices. Since the mechanical properties of the composites are dependent on the compatibility and interaction between the components, improvement of the fiber-matrix interface and interphase interactions in natural fiber/polymer composites is essential. Approaches to improve the interaction and thereby the stress transfer between lignocellulosics and polymer resin include, for instance, the use of chemical or physical modifications on the fiber. The use of chemical modifications on natural fiber has drawn attention as adhesion-promoting treatments for the biocomposites. Surface modification of natural fiber with adequate modifiers has been shown to improve compatibility of the natural fibers with plastics.
Biocomposites, in general are materials made by nature or produced synthetically that include some type of natural material in their structure. Biocomposites are also known as natural fiber composites. Biocomposites are formed through the combination of natural cellulose fibers with other materials such as biopolymers or resins or binders based on renewable raw materials. The need is to combine two or more materials in such a way that a synergism between the components results in a new material that is much better than the individual components. The properties of plant fibers can also be modified through physical and chemical technologies to improve performance of the final bio-composite. Some of the plant fibers with suitable properties for making biocomposites are: hemp, kenaf, coir, henequen, jute, flax, sisal, banana, kapok, etc. Biocomposites can be used for a range of applications, for example, building materials, structural and automotive parts, absorbents, adhesives and bonding agents and degradable polymers. These materials can produce a balance between ecology and economy.
Biocomposites have been traditionally manufactured with techniques like extrusion, injection molding, compression molding, resin transfer molding, pultrusion, and like precursors. Biocomposites have not been produced commercially using a Sheet Molding Compound (SMC) process line. SMC is a very useful processing technique usually used for fabricating glass-polyester resin composites. Today more and more automotive parts to skis are being molded from SMC. This is not too surprising since thermoset composite plastics have been used for more than sixty years around the world. A recent U.S. report states that the demand for thermoset composites in the automotive industry will rise by 68 per cent to 467 million pounds by the middle of this decade. The reason is understandable, SMC combines high stiffness and lighter weight. Other advantages of thermoset composites are durability and resistance to corrosion in a wide range of temperature environments. The application of a priming and topcoat allows SMC components to meet automotive ‘class A’ surface finish requirements and moisture resistance. Processing of SMC by compression molding enables the production of bodywork or structural automotive components and electrical or electronic machine housings in large industrial volumes or body panels (hoods and deck lids). The process also penetrates sectors such as sanitary ware (baths) and urban furniture (stadium and cinema seating), etc. Its characteristics allow SMC to fill a mold under the effects of the molding temperature and pressure.
There have been numerous patents on SMCs. U.S. Pat. No. 3,615,979 granted in 1971 was a patent for glass fiber-reinforced sheet molding compound, by Owens-Corning Fiberglas Corp. U.S. Pat. No. 3,713,927 granted in 1973, thermosetting sheet molding compounds. U.S. Pat. No. 3,835,212 granted in 1974 was a patent for resinous sheet like products. DE 2357000 granted in 1974 was a patent for sheet molding compounds. JP 49076988 granted in 1974 was a patent for unsaturated polyester compositions for moldings. JP 49107086 granted in 1974 was a patent for unsaturated polyester compositions. JP 49107086 granted in 1974 was a patent for unsaturated polyester compositions. JP 50005463 granted in 1975 was a patent for molding of unsaturated polyesters. JP 50008882 granted in 1975 was a patent for molding resin compositions. JP 50014792 granted in 1975 was a patent for sheet molding compositions. JP 50036588 granted in 1975 was a patent for sheet molding compositions. U.S. Pat. No. 3,931,095 granted in 1976 was a patent for fire retardant thermosetting resin composition.
There have been many improvements in the SMC processing since 1970s, and hence many more patents. One patent and two papers talk about SMC processing for biocomposites. These include, US 2003088007 granted in 2003 to Wool et al, which is a patent for sheet molding compound resins from plant oils. van Voorn et al discussed a non automated SMC process for making biocomposites in, “Natural fibre reinforced sheet molding compound”, in 2001.Goswami et al, made jute (cloth) reinforced sheets with the help of polyester resin in “Jute reinforced sheets based on shellac filled SMC”, in 2003.
The traditional SMC is a combination of chopped glass strands and filled polyester resin, in the form of a sheet. SMC is a fully formulated system (fiber, resin, and additives), which can be molded without additional preparation. Sheet molding compound can be made with random chopped fiber reinforcement, with continuous fibers, or with a mixture of the two. The variety of fiber architectures available for sheet molding compound means a variety of properties can be achieved in compression molded parts. Conventional SMC prepreg is made from chopped glass strands, sandwiched between two layers of film, onto which the filled resin paste has already been applied. The prepreg passes through a compaction system that ensures complete strand impregnation before being wound into rolls. These are stored for a few days at reduced temperatures before molding to allow the prepreg to thicken to a moldable viscosity.
SMC is aged to a state near gelation. The manufacturer allows the SMC to reach an extent of reaction close to gelation because it stabilizes the polymer. This light crosslinking increases viscosity and improves processability of the SMC. This ready-to-mold glass fiber reinforced polyester material is primarily used in compression molding. SMC is also mixed with fiber fillers. SMC is made by dispensing mixed resin, fillers, maturation agent, catalyst and mold release agent onto two moving sheets of carrier film. SMC can be molded into complex shapes with little scrap. It is also called the “raw material” for compression molding. It is suitable for production runs in excess of 10,000 parts per year.
Sheet Molding Compound (SMC) produces highly cross-linked and highly filled systems, in which the polymer component is usually an unsaturated polyester. The molded product combines high modulus with high strength. SMC is less expensive than metal, and also has a lower tooling cost.
An industrial process for producing sheet molding compound (biocomposites) is not available. A research group (From Agricultural Research Center, (ATO), Wageingen University Research Center, The Netherlands) has reported the use of SMC for making biocomposites. The natural fibers in this process are not continuously fed to the SMC line. The feeding is by a batch process. Thus, their process is not fully automated, and hence can't be used in an industrial setting. No other process has been reported in literature for fabrication of biocomposites via SMC.